Project Summary The yeast Candida albicans is a prevalent cause of life-threatening systemic disease in the clinic. This is a highly adaptive species with the ability to occupy diverse niches in the human body, either as a benign commensal or as an opportunistic pathogen. The diploid genome consists of eight heterozygous chromosomes that can undergo de novo mutation, loss of heterozygosity (LOH), or larger scale rearrangements including the acquisition or loss of whole supernumerary chromosomes. These mechanisms generate substantial variation in the population, yet there is currently limited understanding of how these natural differences effect interactions with the host. This project will examine how intra-species diversity impacts both the commensal and pathogenic properties of C. albicans. Preliminary experiments reveal that clinical isolates exhibit extensive genotypic differences and that additional microvariation occurs during passaging in the host. To determine how genetic diversity impacts C. albicans biology, a sequenced collection of clinical isolates will be barcoded and analyzed in different murine infection models. We hypothesize that different isolates will show optimal fitness in different host niches, and our studies will reveal those isolates that are hyper- or hypo-competitive for each niche. In parallel, a collection of SC5314 isolates will be barcoded and analyzed for phenotypic and genotypic differences. SC5314 is the standard ?laboratory? isolate of C. albicans and yet preliminary data indicates diversity between isolates obtained from around the world. This reveals a critical need for a detailed analysis of the SC5314 collection to determine the level of genetic variation between strains and how this is affecting phenotypic properties. We will also perform a focused examination of differences in commensalism between two C. albicans isolates, SC5314 and 529L. SC5314 is unable to colonize the murine gastrointestinal tract (GI) in the absence of antibiotics, whereas we reveal that isolate 529L stably maintains GI colonization levels even without antibiotic supplementation. To identify genetic loci responsible for this difference, the two strains have been crossed to one another and recombinant progeny genotyped. These progeny will be used for quantitative trait loci (QTL) mapping to define the loci that underlie GI colonization properties and to understand how genetic variants impact commensalism. Together, these experiments will use high-throughput techniques to examine how genetic diversity in C. albicans populations impacts commensal and pathogenic interactions with the host, as well as a directed approach to examine factors enabling colonization of the GI tract. Our experiments will provide greater understanding of the role that genetic variation in C. albicans plays in infection outcomes, including the identification of mechanisms that promote adaptation to specific host niches.